1. Field of the Invention
The present invention relates to a swash plate type variable displacement pump which drives hydraulic devices and a special-purpose vehicle which has this pump installed therein.
2. Description of the Related Art
Swash plate type variable displacement pumps are widely employed in various industrial machines and industrial vehicles. Japanese Unexamined Utility Model Publication No. 60-19776, for example, discloses one such swash plate type variable pumps. This pump will now be explained referring to FIG. 1.
In the described pump, the opening end of a cup-shaped casing 101 is covered by an end plate 102, thereby forming a crank case 103. A drive shaft 104 that extends into the crank case 103 is supported by the casing 101 and the end plate 102 via bearings 105. A plurality of cylinder blocks 106 are carried by the drive shaft 104 such that they extend in parallel to the drive shaft 104. Thus, the cylinder blocks 106 rotate integrally with the drive shaft 104 inside the crank case 103. Each cylinder block 106 has a bore 107 formed therein. A reciprocable piston 110 is provided in each bore 107. These pistons 110 are coupled to a swash plate 109 through shoes 108.
A valve plate 111 is attached to the end plate 102 adjacent the open end of the bores 107. The valve plate 111 includes an inlet port 112a and a discharge port 112b positioned along the rotational locus of openings 107a of bores 107. The ports 112a and 112b communicate with external hydraulic circuits through an inlet port 113a and a discharge port 113b formed in the end plate 102. The pistons 110 are reciprocated in accordance with the rotation of the cylinder blocks 106, therefore. When a pistons 110 moves away from the end plate (to the left in FIG. 1), it enlarges the volume of the sealed spaces of the bores 107. Thus, a working fluid is sucked in the bores 107 through the inlet port 112a. On the other hand when the pistons 110 move towards the end plate, they reduce the volume of the sealed spaces of the respective bores 107, thereby discharging working fluid from the bores 107 through the discharge port 112b.
The swash plate 109 is supported by a support shaft (not shown), and is urged by a bias spring 114 in a direction to increase the tilting angle. During use, the actions of the pistons 110 themselves tend to urge the swash plate 109 in a direction that decreases the tilting angle. Additionally, a hydraulic cylinder 115 positioned 180.degree. from the swash plate 109 applies a hydraulic positioning force against the swash plate. Accordingly, the actual tilt angle of the swash plate 109, is determined by several combined forces.
In the above-described pump, the spring 114 urges the swash plate 109 in a direction to increase the tilting angle. When the operation of the pump is stopped, pressurized fluid leaks from the cylinder blocks 106 through clearances which are provided to allow the pistons 110 to slide in the cylinder blocks 106 and/or pressurized fluid flows out from a fluid circulation orifice which is provided in the cylinder 115 or its control circuit, dropping the fluid-discharge pressure. As a result, the cylinder 115 loses resisting force, and the swash plate 109 is held still at the maximum angle by the urging force of the spring 114. When the pump is activated again, it will begin operation at its maximum capacity which means that its starting torque will be very large. Thus the initial power consumption will be relatively high and the pump has poor response to large load changes.
The amount of fluid discharged from the pump has to be kept at "0" or close to "0" when the pressurized fluid does not need to be supplied to hydraulic devices. The above-described pump, however, has a limited pressure range of the working fluid that is applied to the cylinder 115, and cannot acquire fluid pressure corresponding to the angle of the swash plate 109 being in the vicinity of 0.degree.. For this reason, with the swash plate 109 held at around 0.degree. C. and the discharge amount set almost to "0, " the operation of the pump cannot continue. It is therefore necessary to provide a clutch between the pump and its input side so as to separate the pump from the input side when the pump is free of a load.
FIG. 2 exemplifies the transmission system of a pump mounted on a special-purpose vehicle, such as a dump truck. As is apparent from FIG. 2, power is transmitted from an engine 120 to a power take-off (PTO) device 122 via a transmission 121, and then sent through a transmission shaft 123 and an electric clutch 124 to drive a pump 125. The structure including the transmission shaft 123 and the clutch 124 complicates the unit and increases the manufacturing cost.
FIG. 3 illustrates another transmission system in which the electric clutch 124 is eliminated, where the pump 125 is driven by turning the power take-off device 122 on and off. This system inevitably requires a complicated operation for turning the PTO device 122 on or off. Specifically, it requires the following steps in order:
(a) switching the drive range of a shift lever by operating a foot brake, PA1 (b) turning the power take-off device 122 on (activating the pump 125), PA1 (d) operating a dump truck (during a work period from the beginning of the work to the end of the work), PA1 (e) switching the drive range of the shift lever by operating the foot brake, and PA1 (f) turning off the power take-off device 122 (deactivating the pump 125).
(c) switching the parking range of the shift lever,